Terrorists struck in the heartland of America yesterday, detonating a powerful car bomb that devastated a federal building in Oklahoma City and killed at least 26 people, including as many as 17 children, and injured hundreds.

Authorities said about 300 people are unaccounted for, with many trapped in the rubble of the nine-story Alfred P. Murrah Federal Building about two miles from the state Capitol. At least 58 people were taken to hospitals with critical injuries.

Fire Chief Gary Marrs said he expected the death toll would be much higher "because we've seen fatalities in the building."

The 9 a.m. blast, which is expected to be the worst terrorist attack on U.S. soil, ripped off a large segment of the building, where more than 500 federal employees were in their offices. It left a crater eight feet deep. Mayor Ron Norick said the blast was caused by a car parked in front of the building that contained more than 1,000 pounds of explosives.

"Obviously, no amateur did this," said Gov. Frank Keating. "Whoever did this was an animal."

A senior law-enforcement source told Newsday that the massive bomb seems to have contained ammonium nitrate, a readily available fertilizer that was also used in the Feb. 26, 1993, World Trade Center bombing, which killed six people and injured 1,000. "That's all we know," he said.

President Bill Clinton called the bombers "evil cowards" and vowed to dispatch the world's best investigators after those responsible. "These people are killers and must be treated like killers," he said. The president said when the suspects are caught "justice will be swift, certain and severe."

Despite various reports about the possible suspects - including three bearded men of Middle Eastern descent driving a pickup truck - officials said they had no evidence linking anyone to the explosion.

"We're pursuing every shred of evidence available," Attorney General Janet Reno said at a press conference late yesterday. And Bob Ricks, who heads the FBI's Oklahoma City office, said, "We have hundreds of potential suspects." Reno said the government, which has dispatched dozens of FBI agents, investigators and explosives experts to Oklahoma City, would seek the death penalty.

For some the debris and cables dangling from the floors brought to mind the car bombings at the U.S. Embassy and Marine barracks in Lebanon in 1983.

"It was like Beirut; everything was burning and flattened," said Dr. Carl Spengler of the city medical examiner's office, who arrived minutes after the blast.

Last night, people were still trapped inside. "We have to crawl on our stomachs and feel our way and we're talking to victims who are in there and reassuring them that we're doing everything within the Lord's power to reach them and set them free," said Assistant Fire Chief Jon Hansen. "It's going to be a very slow process."

The 17 children killed were at a day-care center in the building and ranged from 1 to 7 years old, said Spengler, who was one of the first doctors at the scene. Ten to 20 other children were unaccounted for late in the day.

The center was located on the second floor of the building just above the spot where the bomb exploded, said Faith Wohl, director of the General Services Adminstration that runs the center.

"The day-care center is totally gone," Spengler said.

A state official, who felt the explosion in her building two miles from the explosion, said Oklahomans had felt far removed from the kind of terrorist bombing that rocked New York two years ago.

"We feel so insulated in this part of the country," said Barbara Sieck, admistrative officer for the state's Division of Children and Family Services. "When the Trade Center went down we thought, 'Well that's awful but you know, that's New York.' We just hear those things and think they should never, ever touch us here."

"I never dreamed that anything like this would ever happen in our city," said Mary Lee McCollum, a grocery store clerk who was on a break when she heard the explosion some eight miles away and saw black smoke billowing across the skyline. "It's hard to fathom the idea it would even happen. You don't dream of anyone coming in and invading our lives like this."

In Washington, the president ordered tightened security at all federal buildings after the blast.

Telephone bomb threats led authorities to evacuate a number of government buildings, including in New York. A federal building in Boston and the Boston City Hall were evacuated after doors that should have been locked were found open.

Security at the White House, already tightened because of a shooting there last October, was tightened further, with bags - even lunches - being closely inspected and even X-rayed. Police led bomb-sniffing dogs through the Capitol.

"I just have chills," said a clerk at the federal courthouse in Greeneville, Tenn. "This is just little Greeneville, Tennessee, but it could happen here. We're all very worried."

In Oklahoma City, the bomb blast could be felt 30 miles away.

"It sounded like a sonic boom, only much bigger," said Judy McCullum, who works for Phillips Petroleum Co. a few blocks from the bomb site. She and other employees, under orders to evacuate the building, ran outside to find the streets littered with glass. "It looked like a war zone," she said.

Brian Espe, a state veterinarian, was giving a slide presentation on the fifth floor when the bomb went off. "I dove under the table," he said. "When I came out, I could see daylight if I looked north and daylight if I looked west."

Some 1,500 miles away, John Pitta, the agent in charge of the Long Island office of the Bureau of Alcohol, Tobacco and Firearms, said the disaster represented "an awareness day" for the rest of the country. "In Oklahoma, we've lost a lot of people. It's a tragedy. It's a first . . . The heartland is now aware of what's out there."

Arnold Fagin, a lawyer who was in the county courthouse three blocks away at the time of the blast, said the explosion was "tremendous, like a sonic boom that occurred right over our building, or something like an earthquake."

Fagin said the day-care center tends to 30 to 40 children every day. "That part of the building is just devastated," he said.

In the bomb's aftermath, people were seen frantically searching for loved ones, including parents whose children were in the day-care center.

One of the rescuers, Christopher Wright of the Coast Guard, said searchers frequently turned off chainsaws and drills to listen for calls for help. "But we didn't hear anything, just death," he said. "You're helpless really, when you see people just two feet away, you can't do anything, they're just smashed."

The explosion, which occurred on the second anniversary of the fiery, fatal siege of the Branch Davidian compound in Waco, Texas, hightened fears of terrorism across the country. Federal buildings in several cities were evacuated because of bomb scares and security was tightened in numerous buildings.

Imam Isa Abdul Kareem, chairman of the Long Island Council of Islamic Leadership, last night condemned the bombing. "If it was done by a Muslim, this is not in accordance with the Islamic way," he said. "Islam does not condone the killing of innocent women and children. I would like to extend my sorrow and grief to the relatives of the victims."

The building, which was built in 1974 and named for a former federal judge, houses offices of the Bureau of Alcohol, Tobacco and Firearms, Social Security, Veterans Affairs, the Drug Enforcement Administration and Housing and Urban Development, and a federal credit union and military recruiting offices. It includes an underground parking garage. According to federal officials, 19 DEA, six Secret Service and one AFT employee unaccounted for.

The disaster came 75 years after a bomb blast in New York's Wall Street area that killed 40 people and injured hundreds. Authorities concluded that bombing was the work of "anarchists" and came up with a list of suspects, all of whom had fled to Russia.

To coordinate yesterday's bombing investigation, the FBI has established a command post in Oklahoma City and is in constant contact with the Department of Justice. The FBI has sent four agents to operate the command post and four evidence-response teams and explosive-ordinance teams to Oklahoma City. Fifty more agents are expected in the city today.

"The psychological impact has not even set in yet," said Lt. Gene McPherson of the Oklahoma County Sheriff's Office. "People are just walking around with fixed stares."

McPherson, a Marine Corps veteran who served in Vietnam, said he was moved by the support citizens gave to the rescue efforts. "Citizens were just coming out and helping with anything."

Milissa Kozicki, a spokeswoman for the American Red Cross, said the agency was flooded with inquiries. "People are looking for their children and people called with loved ones in the building . . . A lot of people are just in shock. Why Oklahoma City? Why here?"

"People have been traumatized," said Kozicki. "We have set up a shelter and a place to go for people to share their experiences."

Elizabeth Laurent, a spokeswoman for St. Anthony Hospital, the closest hospital to the bomb scene, said at least 137 people were brought to the hospital for emergency treatment, mostly for cuts caused by flying glass.